Long-lived excited states of zwitterionic copper(I) complexes for photoinduced cross-dehydrogenative coupling reactions

Article abstract of DOI:10.1002/chem.201405356

Four heteroleptic copper(I) complexes containing phenanthroline and monoanionic nido-carborane-diphosphine ligands have been prepared and structurally characterized by various spectroscopic techniques and X-ray diffraction. These complexes exhibit intense

Full text of DOI:10.1002/chem.201405356

DOI: 10.1002/chem.201405356
Full Paper
&
Cross-Dehydrogenative Coupling
Long-Lived Excited States of Zwitterionic Copper(I) Complexes for
Photoinduced Cross-Dehydrogenative Coupling Reactions
Bin Wang,[a] Deepak Prakash Shelar,[b] Xian-Zhu Han,[a] Ting-Ting Li,[b] Xiangguo Guan,[c]
Wei Lu,[d] Kun Liu,[a] Yong Chen,*[b] Wen-Fu Fu,[b] and Chi-Ming Che[c]
Abstract: Four heteroleptic copper(I) complexes containing
phenanthroline and monoanionic nido-carborane-diphos-
phine ligands have been prepared and structurally character-
ized by various spectroscopic techniques and X-ray diffrac-
tion. These complexes exhibit intense absorptions in the visi-
ble range and excited-state lifetimes on the microsecond
scale. Their application in visible-light-induced cross-dehy-
drogenative coupling reactions was investigated. Preliminary
studies showed that one of the four copper(I) complexes is
an efficient catalyst for photoinduced oxidative CÀH func-
tionalization using oxygen as oxidant. Furthermore, a-func-
tionalized tertiary amines were obtained in good-to-excel-
lent yields by light irradiation (l>420 nm) of a mixture of
our CuI complex, tertiary amines, and a variety of nucleo-
philes (nitroalkane, acetone, or indoles) under aerobic condi-
tions. Electron paramagnetic resonance measurements pro-
vided evidence for the formation of superoxide radical
anions (O2À·) rather than singlet oxygen (1O2) during these
photocatalytic reactions.
Introduction
cient catalysts for aerobic oxidative CÀH bond functionaliza-
tion. However, these metals are expensive and scarce, which is
disadvantageous for possible large-scale applications. The re-
placement of ruthenium and iridium by earth-abundant metals
would be highly desirable. One promising alternative is
copper, in particular, the excited-state properties of copper(I)
phenanthroline complexes are comparable to the well-known
[Ru(bpy)3]2+ (bpy=2,2’-bipyridine).[6] However, compared with
ruthenium and other noble metal complexes, the use of cop-
per(I) complexes in the fields of both synthetic photocatalysis
and solar energy conversion lags far behind, probably because
copper(I) complexes usually suffer from some drawbacks. First,
copper(I) diimine complexes are apt to be oxidized to copper(-
II), which limits their practical applications. Secondly, the emis-
sion from the charge-transfer excited state of copper(I) com-
plexes is typically weak and short-lived due to the formation of
exciplexes and severe structural distortion in the excited
states.[6,7] To retard the oxidation process and stabilize the cop-
per(I) center, mixed-ligand systems involving a chelating phos-
phine other than a nitrogen ligand have been developed.[8] On
the other hand, disubstitution at the 2,9-positions of the phe-
nanthroline ligand, to some extent, can improve emission effi-
ciency by suppressing nonradiative decay and exciplex
quenching.[9] Despite these advances, it is still a formidable
challenge to achieve robust copper(I) complexes that can be
used as efficient sensitizers in photoredox transformations.[10,11]
With our continuing interest in developing robust copper(I)
complexes[12] and in harnessing long-lived triplet excited states
for visible-light photoredox catalysis,[5,13] herein we report on
four zwitterionic copper(I) complexes Cu1–Cu4 possessing
mixed phenanthroline and nido-carborane-diphosphine ligands
(Scheme 1) as photocatalysts for CDC reactions. The anionic
Recently, there has been an upsurge of interest in the use of
visible light to initiate cross-dehydrogenative coupling (CDC)
reactions.[1] MacMillan[2] and Stephenson[3] and their co-workers
published seminal works on visible-light-promoted CÀC bond
formation catalyzed by ruthenium or iridium polypyridyl com-
plexes as photoredox catalysts. In addition, platinum[4] and
gold[5] complexes have also been reported to act as highly effi-
[a] Dr. B. Wang,+ X.-Z. Han, Dr. K. Liu
Tianjin Key Laboratory of
Structure and Performance for Functional Molecules
Key Laboratory of
Inorganic-Organic Hybrid Functional Materials Chemistry
Ministry of Education, College of Chemistry
Tianjin Normal University, Tianjin 300387 (China)
[b] Dr. D. P. Shelar,+ T.-T. Li, Prof. Dr. Y. Chen, Prof. Dr. W.-F. Fu
Key Laboratory of
Photochemical Conversion and Optoelectronic Materials
Technical Institute of Physics and Chemistry
Chinese Academy of Sciences, Beijing 100190 (China)
Fax: (+86)10-6255-4670
E-mail: chenyong@mail.ipc.ac.cn
[c] Dr. X. Guan, Prof. Dr. C.-M. Che
State Key Laboratory of Synthetic Chemistry
HKU-CAS Joint Laboratory on New Materials
Department of Chemistry, The University of Hong Kong
Pokfulam Road, Hong Kong (China)
[d] Prof. Dr. W. Lu
Department of Chemistry
South University of Science and Technology of China
Shenzhen, Guangdong 518055 (China)
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201405356.
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Scheme 1. Synthesis and chemical structures of Cu1–Cu4.
nido-carborane-diphosphine ligand is expected to exhibit
strong CuÀP bonds and its steric bulkiness can prohibit distor-
tion of the excited-state geometry, thus leading to a long-lived
excited state. We note that Yersin et al. have described the syn-
thesis of these zwitterionic complexes in their recent patent[14]
in which CuI complexes were harnessed as light emitters in or-
ganic electronic devices. However, neither detailed photophysi-
cal properties nor photocatalytic applications of these CuI com-
plexes were reported. This study has revealed that Cu1–Cu4
have a distinct low-energy absorption band in the 370–550 nm
region. As expected, the bulky complexes Cu2 and Cu3 also
display an intense and relatively long-lived emission, which is
desirable for use in light–chemical energy conversion. Indeed,
we have demonstrated that these complexes can be used to
catalyze the oxidative CÀC coupling of CÀH bonds with
oxygen as the oxidant under visible-light irradiation. Further-
more, superoxide radical anions (O2À·) have been found to play
a crucial role in this photoredox system.
Figure 1. X-ray crystal structure of Cu2. Perspective view with thermal ellip-
soids drawn at the 30% probability level. Hydrogen atoms and solvent mol-
ecules have been omitted for clarity.
tion. Briefly, the central copper atom in Cu2 is coordinated by
two nitrogen and two phosphorus atoms in a slightly distorted
tetrahedral geometry. The CuÀN bond lengths of 2.0730(2)/
2.1177(2) ꢁ and CuÀP distances of 2.2659(6)/2.2719(7) ꢁ are
normal for heteroleptic copper(I) complexes containing mixed
nitrogen and phosphine ligands. However, the PÀCuÀP angle
of 90.44(2)8 in Cu2 is markedly smaller than that of 116.44(4)8
in [Cu(dmp)(pop)]+ (dmp=2,9-dimethyl-1,10-phenanthroline;
pop=bis[2-(diphenylphosphino)phenyl] ether) and 122.7(1)8 in
[Cu(dmp)(PPh3)2]+.[8,16]
The photophysical properties of Cu1–Cu4 were examined
under various conditions, and the data are presented in
Table 1. As shown in Figure 2, the absorption spectra of Cu1–
Cu4 in dichloromethane each display an intense peak at 271–
285 nm and a broad band in the 370–550 nm region. The
high-energy absorptions should be associated with both nitro-
gen and phosphine ligands, and the low-energy absorptions
are mainly attributed to metal-to-ligand charge-transfer transi-
tions (MLCT) according to density functional theory calcula-
tions (see Figure S5 in the Supporting Information). The CT ab-
sorptions are blueshifted upon going from Cu1 to Cu2 as
Results and Discussion
The 1,2-bis(diphenylphosphino)carborane ligand was synthe-
sized by the reaction of carborane with diphenylchlorophos-
phine in the presence of butyllithium as base.[15] Complexes
Cu1–Cu4 were prepared in methanol or ethanol by treating
[Cu(MeCN)4]BF4 with the carborane-diphosphine and, subse-
quently, the appropriate phenanthroline ligands; this proce-
dure is slightly different from that of Yersin and co-workers.[14]
During this reaction, partial degradation of the neutral closo-
carborane-diphosphine [(PPh2)2C2B10H10] took place and afford-
ed an anionic nido-carborane-diphosphine [(PPh2)2C2B9H10]À,
which leads to the formation of zwitterionic copper(I) com-
1
plexes. All complexes were fully characterized by H, 13C, and
31P NMR spectroscopy, FAB mass spectrometry, IR spectroscopy,
1
and elemental analysis. The H NMR spectra of Cu1–Cu4 show
one broad signal at around À2.0 ppm, which can be ascribed
to BÀHÀB. Detailed experimental procedures and characteriza-
tion data are provided in the Experimental Section.
Single crystals of Cu2 and Cu4 suitable for X-ray diffraction
analysis were grown by vapor diffusion of diethyl ether into di-
chloromethane solutions of the complexes. The crystal struc-
ture of Cu2 is depicted in Figure 1 and detailed crystallograph-
ic data are summarized in Table S1 in the Supporting Informa-
Figure 2. UV/vis absorption (in CH2Cl2, c=2ꢂ10À5 moldmÀ3) and solid-state
emission spectra (at room temperature, lex =365 nm) of Cu1–Cu4.
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cate that they are powerful photo-oxidants. Then we
screened their use in visible-light-induced cross-dehy-
drogenative coupling reactions. The results of the
CDC reaction of 1a with nitromethane with Cu1–Cu4
as the visible-light photoredox catalyst are summar-
ized in Table 2. The results show that Cu3 has the
best catalytic activity (Table 2, entries 1–4). Interest-
ingly, a yield of 10% of the desired product 3a was
obtained in the absence of any catalyst (Table 2,
entry 5); however, no conversion was observed when
the reaction was conducted in the dark (Table 2,
entry 6). Furthermore, only trace amounts of produc-
t 3a were obtained when the CÀC coupling reaction
was carried out under argon instead of molecular
oxygen (Table 2, entry 7). When oxygen was replaced
by air, the system was also effective, but a slightly
Table 1. Photophysical data for Cu1–Cu4.
[b]
[c]
[d]
[e]
Comp. labs[a] [nm]
lem [nm]
lem [nm] lem [nm]
lem [nm]
(e [dm3 molÀ1cmÀ1]) (t [ms]; f [%]) (t [ms])
(t [ms]; f [%]) (t [ms])
Cu1
271 (32090)
322 (9430)
465 (5360)
274 (28340)
438 (3030)
285 (40260)
449 (4630)
285 (42560)
473 (6900)
non-emissive 610 (170) 600 (1.8; 1.8) 633 (37.3)
Cu2
Cu3
Cu4
592 (1.2; 5.0) 550 (1509) 567 (8.7; 34) 566 (1010)
602 (1.4; 4.9) 566 (1645) 586 (7.0; 28) 586 (1310)
non-emissive 620 (265) 632 (1.5; <1) 670 (29.7)
[a] Measured in CH2Cl2 solution at 298 K (c=2ꢂ10À5 moldmÀ3). [b] Measured in de-
gassed CH2Cl2 solution at 298 K (c=2ꢂ10À5 moldmÀ3). [c] Measured in butyronitrile at
77 K. [d] Measured in the solid state at 298 K. [e] Measured in the solid state at 77 K.
a result of the incorporation of electron-donating methyl
groups at the 2,9-positions of phenanthroline, but redshifted
from Cu2 to Cu3 and from Cu1 to Cu4 following the introduc-
tion of phenyl rings at the 4,7-positions of phenanthroline due
to a lowering of the LUMO energy levels. It is notable that the
CT bands of Cu1–Cu4 are shifted towards longer wavelengths
than those of [Cu(phen)(pop)]+ (phen=1,10-phenanthroline),
which could be a result of the strong electron-donating char-
acter of the phosphine ligand and its small PÀCuÀP bite
angle.[17]
lower yield was obtained (Table 2, entry 8). All the above re-
sults suggest that light irradiation and molecular oxygen are
essential for the reaction and that the copper(I) complexes, es-
pecially Cu3, as photocatalysts facilitate CÀC bond formation.
Thus, the optimal reaction conditions include the irradiation of
nitromethane and 1a in the presence of 1.5 mol% of Cu3
under oxygen for 8 h.
To explore the scope of the oxidative aza-Henry reactions of
tertiary amines, a variety of N-aryltetrahydroisoquinolines 1a–
i were subjected to the optimized reaction conditions; the de-
sired coupling products 3a–i were obtained in good-to-excel-
lent yields (Table 3, entries 1–9). The introduction of both elec-
tron-donating and -accepting groups on the phenyl ring has
little effect on the reaction yields. Furthermore, the use of ni-
troethane and -propane as nucleophile also provided the de-
sired products 3j–o but in lower yields (Table 3, entries 10–15),
which demonstrates that the reaction is significantly affected
by the size of the nucleophile.
The photoluminescence of the copper(I) phenanthroline
complexes is strongly dependent on the steric effects of the li-
gands. In general, decoration of the phenanthroline at the 2,9-
positons with alkyl or phenyl groups will give rise to a notable
increase in the emission intensity by suppressing the structural
distortion from tetrahedral to square planar in the excited
state.[6] In the present systems, Cu2 and Cu3 with methyl
groups at the 2,9-positions exhibit efficient emissions with lmax
(lifetime; quantum yield) at 592 (1.2 ms; 5.0%) and 602 nm
(1.4 ms; 4.9%), respectively. In sharp contrast, Cu1 and Cu4
with no substituents at these positions are nonemissive in di-
chloromethane at room temperature, consistent with the ob-
servations for the [Cu(phen)(pop)]+ system.[8] Upon cooling to
77 K, the emission maxima of Cu2 and Cu3 shift to higher en-
ergies, accompanied by a dramatic increase in emission life-
time from micro- to milliseconds. In the solid state, all the
complexes show broad and unstructured emissions, character-
istic of a charge-transfer state (Figure 2). At both room temper-
ature and 77 K, the maximum emission wavelengths for Cu2
and Cu3 are located at 567 and 586 nm, respectively, whereas
those for Cu1 and Cu4 are shifted towards lower energies as
the temperature decreases. The redshift in emission energy
and marked increase in emission lifetime suggest the involve-
ment of thermal-activated delayed fluorescence.[18]
Table 2. Optimization of the CDC reaction of tetrahydroisoquinoline 1a
with nitromethane.
Entry
Conditions[a]
Cu1 (1.5 mol%)
Conv. [%][b]
Yield [%][c]
1
2
3
4
5
6
7[e]
8
25
100
100
29
12
0
18
78
91 (87)(d)
23
10
0
trace
63
Cu2 (1.5 mol%)
Cu3 (1.5 mol%)
Cu4 (1.5 mol%)
no catalyst
no light, Cu3 (1.5 mol%)
Cu3 (1.5 mol%)
2
75
Cu3 (1.5 mol%) in air
The electrochemical properties of these copper(I) complexes
in dichloromethane were measured by cyclic voltammetry at
298 K. The voltammograms are depicted in Figure S6 in the
Supporting Information, and the electrochemical data are
listed in Table S2. Excited-state reduction potentials of +0.72 V
for Cu2 and +1.01 V for Cu3 were estimated by combining
the electrochemical and spectroscopic data; the values indi-
[a] Reagents and conditions: 1a (0.2 mmol), CH3NO2 (10 mL). The reac-
tion was saturated with O2 before irradiation unless indicated otherwise;
1
reaction time 8 h. [b] Conversions were determined by H NMR analysis of
the crude by using 4,4’-dimethyl-2,2’-bipyridine as an internal standard
and are based on 1a. [c] The yields are given on the basis of the starting
1
amount of 1a and were determined by H NMR analysis. [d] Isolated yield
based on 1a. [e] The reaction was carried out under argon.
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coupling constants are in agreement with the report-
Table 3. CDC reaction of tetrahydroisoquinolines 1a–i with nitroalkanes 2a–c by
using Cu3 as photocatalyst.[a]
À·
ed values for the adduct of O2 with DMPO.[20] By
1
contrast, when TEMP, an O2 scavenger, was used in-
stead of DMPO in the same air-saturated solution,
the nitroxide radical TEMPO was barely detected (Fig-
ure 3b), whereas in the absence of 1a this EPR signal
is clearly observed (Figure 3c).[21] These observations
À·
imply that O2 plays a key role in this photoredox
system.
Entry Product
Yield Entry Product
[%][b]
Yield
[%][b]
On the basis of the above results and previous re-
ports,[19] we propose the following reaction pathway
(Scheme 2). It is believed that, upon visible-light exci-
tation, a single electron transfer (SET) from 1a to the
excited Cu3* results in the generation of radical
cation 4 and radical anion Cu3À·. Subsequent elec-
tron transfer from Cu3À· to molecular oxygen occurs
to generate the superoxide radical anion (O2À·) and
Cu3 is simultaneously recovered. Then, hydrogen ab-
1
87
85
79
80
9
76
74
70
62
2
3
4
10
11
12
À·
straction of 4 by O2 gives the hydroperoxide HOO·
radical and intermediate 5. Then, the reaction of the
hydroperoxide HOO· radical with 5 furnishes the hy-
droperoxide intermediate 6, which may undergo pro-
tonation to produce the iminium ion 7 along with
the formation of H2O2.[22] On the other hand, single
electron transfer from radical 5 to the triplet Cu3*
also produces iminium ion 7 and the Cu3À· radical
anion. Finally, the nucleophilic addition of the azinic
acid tautomer of nitromethane to iminium 7 fur-
nishes the desired product 3a.
After gaining an understanding of the reaction
mechanism, the present photocatalytic CDC method
was extended to the coupling of tetrahydroisoquino-
lines and acetone. Thus, when a mixture of N-arylte-
trahydroisoquinoline, acetone, and Cu3 in the pres-
ence of a co-catalyst, l-proline, was irradiated with
visible light under an oxygen atmosphere, the de-
sired Mannich-type products were obtained in mod-
erate yields (Table 4). Similarly, the photocatalytic
aerobic CDC reaction was also applied to the cou-
pling of N-aryltetrahydroisoquinolines and indoles. By
using a combination of photocatalyst Cu3 and FeSO4
the desired cross-coupling products were obtained in
good yields (Table 5).
5
6
7
8
78
75
82
80
13
14
15
66
58
48
[a] Reagents and conditions: 1 (0.2 mmol), Cu3 (0.003 mmol), nitroalkane (10 mL), ir-
radiated at ambient temperature in the presence of oxygen. [b] Isolated yields based
on substrate 1.
To gain further insight into the mechanism of the copper-
Conclusion
catalyzed CDC reaction, an EPR experiment was carried out. It
has been reported that both the superoxide radical anion
(O2À·) and singlet oxygen (1O2) may be the active species in the
CDC reaction.[13,19] Thus, we studied the active species of
oxygen in the photocatalytic Cu3 system by using 5,5-dimeth-
yl-1-pyrroline N-oxide (DMPO) and 2,2,6,6-tetramethylpiperi-
Four zwitterionic copper(I) complexes with phenanthroline and
charge-negative nido-carborane-diphosphine ligands have
been prepared. These heteroleptic complexes exhibit intense
visible absorptions and long-lived excited states. Furthermore,
the copper(I) complexes bearing methyl groups at the 2,9-posi-
tions of the phenanthroline ligand efficiently photocatalyze the
oxidative CÀC coupling of CÀH bonds with oxygen as oxidant.
This work highlights the potential of copper(I) complexes in
synthetic photocatalysis and solar-energy conversion that de-
serves further research endeavors.
À·
dine (TEMP) as scavenger for O2 and 1O2, respectively. As
shown in Figure 3a, irradiation of an air-saturated DMF solution
of DMPO, Cu3, and 1a by a 100 W mercury lamp with a filter
to cut off light below 420 nm resulted in the formation of the
characteristic signal of O2À·; the spectrum and the hyperfine
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Table 5. CDC reaction of tetrahydroisoquinolines 1a–c with indoles
9a,b.[a]
Product (yield [%])[b]
Figure 3. a) EPR spectrum of a solution of Cu3 (1.0ꢂ10À4 molLÀ1), 1a
(1.5ꢂ10À3 molLÀ1), and DMPO (2.0ꢂ10À2 molLÀ1) in air-saturated DMF upon
irradiation for 120 s. b) EPR spectrum of a solution of Cu3
(1.0ꢂ10À4 molLÀ1), 1a (1.5ꢂ10À3 molLÀ1), and TEMP (0.12 molLÀ1) in air-sa-
turated DMF upon irradiation for 120 s. c) EPR spectrum of a solution of Cu3
(1.0ꢂ10À4 molLÀ1) and TEMP (0.12 molLÀ1) upon irradiation for 120 s in air-
saturated DMF.
[a] Reagents and conditions: 1 (0.2 mmol), Cu3 (0.003 mmol), indoles
(0.4 mmol), FeSO4 (0.4 mmol), dissolved in DMF (4 mL), irradiated at ambi-
ent temperature in the presence of oxygen. [b] Isolated yields based on
substrate 1.
Experimental Section
Electrospray ionization (ESI) mass spectra were recorded on a Finni-
gan LCQ quadrupole ion trap mass spectrometer (samples were
dissolved in HPLC-grade methanol). Fast atom bombardment (FAB)
mass spectra were recorded on a Finnigan MAT 95 mass spectrom-
1
eter by using 3-nitrobenzyl alcohol as the matrix. H, 13C, 19F, and
31P NMR spectra were recorded using
a Bruker Avance DPX
400 MHz instrument. Elemental analyses were performed on
a Vario EL III instrument at the Beijing Institute of Chemistry, Chi-
nese Academy of Sciences. IR spectra were recorded on a Bio-Rad
FT-IR spectrometer. UV/Vis absorption spectra were recorded on
a Perkin-Elmer Lambda 19 UV/vis spectrophotometer. Steady-state
emission spectra were recorded on a SPEX 1681 Fluorolog-2 Model
F111 fluorescence spectrophotometer equipped with a Hamamatsu
R928 PMT detector. Emission lifetime measurements were per-
formed with a Quanta-Ray Q-switch DCR-3 pulsed Nd:YAG laser
system (pulse output 355 nm, 8 ns). Luminescent quantum yields
were referenced to degassed [Ru(bpy)3](ClO4)2 in acetonitrile (Fr =
0.062) with an estimated error of Æ15%. To measure the absolute
solid-state emission quantum yields, an integrating sphere F-3018
(HORIBA JOBIN YVON) was attached to the SPEX 1681 Flurolog-2
Model F111 spectrophotometer.
Scheme 2. Proposed reaction pathways for the aerobic photocatalytic CDC
reaction catalyzed by Cu3.
Table 4. Mannich reaction of tetrahydroisoquinolines 1a–c with aceto-
ne.[a]
EPR spectroscopy: EPR spectra were recorded at room tempera-
ture by using a Bruker ESP-300E spectrometer at 9.8 GHz, X-band
with 100 Hz field modulation. The samples were quantitatively in-
jected into quartz capillaries especially made for EPR analysis
before being purged with argon or oxygen for 30 min in the dark
and illuminated directly in the cavity of the EPR spectrometer with
a 100 W mercury lamp.
Product (yield [%])[b]
X-ray diffraction: Single-crystal X-ray diffraction analyses were car-
ried out on a Bruker Smart Apex II CCD diffractometer with graph-
ite-monochromated MoKa radiation (l=0.71073 ꢁ) by using f/w
scan technique at room temperature. Semi-empirical absorption
corrections were applied by using SADABS.[23] All structures were
[a] Reagents and conditions: 1 (0.2 mmol), Cu3 (0.003 mmol), l-proline
(0.04 mmol), acetone (6 mL), CH3OH (4 mL), irradiated at ambient temper-
ature in the presence of oxygen. [b] Isolated yields based on substrate 1.
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solved by direct methods using the SHELXS program of the
SHELXTL package and refined with SHELXL.[24]
[Cu(2,9-dimethylphen){(PPh2)2C2B9H10}] (Cu2): Ethanol was used
as solvent. Yield: 0.15 g (64%); IR (KBr): n˜ =2516 cmÀ1 (BÀH);
1H NMR (400 MHz, [D6]DMSO): d=8.83 (d, J=8.2 Hz, 1H; phen),
8.47 (d, J=8.2 Hz, 1H; phen), 8.19–8.14 (m, 2H; phen), 8.07 (d, J=
8.6 Hz, 1H; phen), 7.38–7.28 (m, 1H; phen, 20H; PPh2), 3.39 (s, 3H;
CH3), 0.95 (br, B-H), 0.54 (s, 3H; CH3), À1.86 ppm (br, B-H-B); 31P{1H}
NMR (162 MHz, [D6]DMSO): d=15.15 ppm; MS (FAB): m/z: 773.2
[M]+; elemental analysis calcd for C40H42B9CuN2P2·2H2O, 809.60: C
59.34, H 5.73, N 3.46; found: C 59.56, H 5.39, N 3.59.
Theoretical calculations: Density functional theory (DFT) and time-
dependent density functional theory (TDDFT) calculations were
performed by using the Gaussian 09 package[25] to gain an under-
standing of the geometries and electronic structures of the cop-
per(I) complexes Cu1–Cu4. PBE0[26]/6-31G*(lanl2dz)[27] was used for
the geometry optimization and PBE0/6-31+G* was used for the
UV/Vis calculations. Solvent effects were studied by using the self-
consistent reaction field (SCRF) method based on PCM models.[28]
The choice of solvent (dichloromethane, dielectric constant=8.93)
was based on the solvent used for the experiments.
[Cu(2,9-dimethyl-4,7-diphenylphen){(PPh2)2C2B9H10}] (Cu3): Etha-
nol was used as solvent. Yield: 0.16 g (57%); IR (KBr): n˜ =2526 cmÀ1
1
(BÀH); H NMR (400 MHz, CDCl3): d=7.95 (d, J=9.3 Hz, 1H; phen),
7.90 (s, 1H; phen), 7.87 (d, J=9.3 Hz, 1H; phen), 7.62–7.29 (m,
21H; Ph), 7.22–7.18 (m, 9H; Ph), 7.07 (s, 1H; phen), 3.47 (s, 3H;
CH3), 0.71 (s, 3H; CH3), À1.73 ppm (br, B-H-B); 31P{1H} NMR
(162 MHz, CDCl3): d=15.99 ppm; MS (FAB): m/z: 925.3 [M]+; ele-
mental analysis calcd for C52H50B9CuN2P2·2CH2Cl2·2H2O, 1131.66: C
57.31, H 5.17, N 2.48; found: C 57.63, H 5.08, N 2.20.
Electrochemistry: Cyclic voltammetry (CV) experiments were per-
formed with a computer-controlled CHI660C electrochemical work-
station. In a homogeneous system, CV was carried out in dichloro-
methane containing a glassy carbon electrode (diameter: 3 mm) as
the working electrode, a platinum slice as the counter electrode,
and an Ag/AgCl electrode as the reference electrode. Tetrabutylam-
monium hexafluorophosphate (0.10 molLÀ1) was used as the sup-
porting electrolyte. Electrochemical measurements were conducted
[Cu(4,7-diphenylphen){(PPh2)2C2B9H10}] (Cu4): Methanol was used
as solvent. Yield: 0.16 g (59%); IR (KBr): n˜ =2521 cmÀ1 (BÀH);
1H NMR (400 MHz, CDCl3): d=9.83 (d, J=5.0 Hz, 1H; phen), 8.08–
8.06 (m, 2H; phen), 8.00 (d, J=9.4 Hz, 1H; phen), 7.64–7.18 (m,
31H; phen, Ph), 6.73 (d, J=4.9 Hz, 1H; phen), À1.80 ppm (br, B-H-
B); 31P{1H} NMR (162 MHz, CDCl3): d=18.59 ppm; MS (FAB): m/z:
under N2 at a scan rate of 100 mVsÀ1
.
Commercially available reagents and solvents were used without
further purification unless indicated otherwise. The solvents used
for the photophysical measurements were of HPLC grade. All the
N-aryltetrahydroisoquinolines 1 needed for the CDC reactions were
prepared by using the reported procedure[29] and purified by
column chromatography.
897.3
[M]+;
elemental
analysis
calcd
for
C50H46B9CuN2P2·CH2Cl2·2H2O, 1018.67: C 60.13, H 5.15, N 2.75;
found: C 60.26, H 4.94, N 2.72.
General procedure for the CDC reaction of nitroalkanes with tet-
Synthesis of 1,2-bis(diphenylphosphino)carborane: nBuLi (1.6m
in hexane, 7 mL, 11.2 mmol) was added to a solution of ortho-car-
borane (0.72 g, 5 mmol) in diethyl ether at 08C and the mixture
was stirred for 30 min at room temperature and then heated at
reflux for 30 min. Diphenylchlorophosphine (2.3 mL, 12.5 mmol)
was added to the mixture dropwise at 08C and the reaction mix-
ture was allowed to warm up to room temperature for 30 min
with stirring. The reaction was quenched with a saturated aqueous
ammonium chloride solution and the mixture extracted with dieth-
yl ether, washed with saturated aqueous sodium chloride, dried
over anhydrous MgSO4, and then concentrated. Purification by
column chromatography on silica gel gave the 1,2-bis(diphenyl-
rahydroisoquinolines:
a rubber septum and magnetic stirring bar was charged with Cu3
(2.7 mg, 0.003 mmol), 2-phenyl-1,2,3,4-tetrahydroisoquinoline
A 15 mL Pyrex tube equipped with
(42 mg, 0.2 mmol), and CH3NO2 (10 mL). A stream of oxygen was
bubbled through the mixture for 30 min. The tube was then sealed
and irradiated with a 300 W xenon lamp at ambient temperature.
A glass filter was employed to cut off light with a wavelength
below 420 nm. The progress of the reaction was monitored by TLC
at regular intervals. After 8 h of irradiation, the solvent was re-
moved under vacuum and the residue purified by column chroma-
tography on silica gel to afford the corresponding products.
General procedure for the Mannich reaction of acetone with tet-
1
phosphino)carborane ligand (1.56 g, 61%) as a white solid. H NMR
(400 MHz, CDCl3): d=7.68–7.62 (m, 8H), 7.42–7.35 (m, 12H), 3.0–
1.4 ppm (br, 10H); 31P{1H} NMR (162 MHz, CDCl3): d=7.23 ppm.
rahydroisoquinolines:
a rubber septum and magnetic stirring bar was charged with Cu3
(2.7 mg, 0.003 mmol), 2-phenyl-1,2,3,4-tetrahydroisoquinoline
A 15 mL Pyrex tube equipped with
General procedure for the synthesis of copper(I) complexes
Cu1–Cu4: [(PPh2)2C2B10H10] (0.15 g, 0.3 mmol) was added to a sus-
pension of [Cu(MeCN)4]BF4 (0.09 g, 0.3 mmol) in methanol or etha-
nol (20 mL). The mixture was stirred at room temperature for
30 min and then a solution of the diimine ligand (0.3 mmol) was
added. The reaction mixture was heated at reflux for 1 h. During
this time the complex formed as a precipitate, which was filtered
by suction, washed with n-hexane, and dried under vacuum. Vapor
diffusion of diethyl ether into the CH2Cl2 solution of these com-
plexes afforded single crystals.
(42 mg, 0.2 mmol), l-proline (4.6 mg, 0.04 mmol), acetone (6 mL),
and CH3OH (4 mL). A stream of oxygen was bubbled through the
mixture for 30 min. The tube was then sealed and irradiated with
a 300 W xenon lamp at ambient temperature. A glass filter was
employed to cut off light with a wavelength below 420 nm. After
8 h of irradiation, the solvent was removed under vacuum and the
residue was purified by column chromatography on silica gel to
afford the corresponding products.
General procedure for the CDC reaction of indole with tetrahy-
droisoquinolines: A 15 mL Pyrex tube equipped with a rubber
septum and magnetic stirring bar was charged with Cu3 (2.7 mg,
[Cu(phen){(PPh2)2C2B9H10}] (Cu1): Methanol was used as solvent.
Yield: 0.16 g (71%); IR (KBr): n˜ =2535 cmÀ1 (BÀH); 1H NMR
(400 MHz, [D6]DMSO): d=9.90 (s, 1H; phen), 8.92 (d, J=8.2 Hz, 1H;
phen), 8.61 (d, J=8.0 Hz, 1H; phen), 8.35–8.32 (m, 1H; phen), 8.25
(d, J=9.0 Hz, 1H; phen), 8.17 (d, J=8.7 Hz, 1H; phen), 7.41–7.29
(m, 1H; phen, 20H; PPh2), 6.42 (s, 1H; phen), 0.62 (br, B-H),
À1.92 ppm (br, B-H-B); 31P{1H} NMR (162 MHz, [D6]DMSO): d=
17.95 ppm; MS (FAB): m/z: 745.2 [M]+; elemental analysis calcd
for C38H38B9CuN2P2·2H2O, 781.54: C 58.40, H 5.42, N 3.58; found:
C 58.58, H 5.20, N 3.39.
0.003 mmol),
2-phenyl-1,2,3,4-tetrahydroisoquinoline
(42 mg,
0.2 mmol), indole derivatives (0.4 mmol), FeSO4 (0.4 mmol), and
DMF (4 mL). A stream of oxygen was bubbled through the mixture
for 30 min. The tube was then sealed and irradiated with a 300 W
xenon lamp at ambient temperature. A glass filter was employed
to cut off light with a wavelength below 420 nm. After 8 h of irra-
diation, the solvent was removed under vacuum. Then diethyl
ether (25 mL) and water (25 mL) were added to the residue, and
the organic layer was extracted with diethyl ether (3ꢂ25 mL). The
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sodium sulfate. The solvent was removed by rotary evaporation
and purified by column chromatography on silica gel to afford the
corresponding products.
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This work was supported by the 973 Program (2013CB834800
and 2013CB632403), the Ministry of Science and Technology of
China (2012DFH40090), the NSFC (21102101, 21172173,
21273257, and 21371175). We thank Beijing Natural Science
Foundation (2142033), Tianjin Normal University Foundation
(5L105), and the CAS-Croucher Funding Scheme for Joint Labo-
ratories. D.P.S. and Y.C. acknowledge the support of the Chi-
nese Academy of Sciences Visiting Fellowship for Researchers
from Developing Countries (2013FFGB0009) and 100 Talents
Program, respectively.
Keywords: carboranes
·
copper
·
cross-coupling
·
photocatalysis · UV/Vis spectroscopy
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Received: September 21, 2014
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Published online on && &&, 0000
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FULL PAPER
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Cross-Dehydrogenative Coupling
B. Wang, D. P. Shelar, X.-Z. Han, T.-T. Li,
X. Guan, W. Lu, K. Liu, Y. Chen,* W.-F. Fu,
C.-M. Che
&& – &&
Long-Lived Excited States of
Zwitterionic Copper(I) Complexes for
Photoinduced Cross-Dehydrogenative
Coupling Reactions
Oxidative cross-coupling: The long-
lived excited states of heteroleptic cop-
per(I) complexes containing phenan-
throline and monoanionic nido-carbor-
ane-diphosphine ligands catalyze oxida-
tive CÀC coupling reactions with
oxygen as the oxidant under visible-
light irradiation (see figure).
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!